Thanks again for your ideas.

The problem is really what type of coupling to use to resist the
torque forces of the tiller. If I could use flanges, the total
x-section area of the bolts that hold the flanges together should be
the equivalent to the x-section area of the rudder post, so there is
the potential for a lot of torque. As you said, splines may losen and
wear, set screws may wonder, and even the bolting method that you
describe may not be stong enough. Once a little movement occurs there
is accelerated wear and eventual failure.

I did a search for mechanical couplings and found a wealth of
resources; now it is a matter of sifting through them to see what I
can come up with. There is even have one device that resists torque
forces by heat-shrinking the coupling to the shaft!

Cheers, Jim